COVID-19 vaccines can add an extra day to women’s menstrual cycle

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BETHESDA, Md. ( – COVID vaccinations can temporarily prolong a woman’s menstrual cycle, a new study shows. Researchers found that women who received a dose of a coronavirus vaccine during their menstrual cycle found that their cycle increased by up to one day compared to unvaccinated women.

The study, funded by the National Institutes of Health (NIH), finds that the changes in cycles affect the time between bleedings, but not the bleeding period itself.

Still within the ‘normal’ range?

Research authors, led by Dr. Alison Edelman of Oregon Health & Science University in Portland, confirmed that it is not uncommon for menstrual cycles to vary from month to month, and the increase they found was “well within normal” variability. Edelman adds that further research is needed to determine how COVID vaccines can affect other menstrual characteristics such as pain, mood swings and the severity of flow.

“It is reassuring that the study found only a small, temporary menstrual change in women,” says Dr. Diana Bianchi, director of the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development, in a media release.

“These results provide, for the first time, an opportunity to advise women on what to expect from COVID-19 vaccination so that they can plan accordingly.”

Bianchi notes that there has been little research into how COVID vaccines or vaccines against other diseases can potentially affect the menstrual cycle. In an attempt to understand why the changes occur, the team analyzed data from the fertility tracking app Natural Cycles.

To get an accurate reading, users need to enter data about their temperature and menstrual cycles. For vaccinated women, the researchers looked at data from three consecutive cycles before the patients received the vaccine, and from a further three consecutive cycles after – including the cycle or cycles in which their vaccination took place. For unvaccinated participants, the researchers collected their data from six consecutive cycles.

The menstrual cycle length can in some cases last up to 2 days longer

Of the 3,959 people in the study, 2,403 were vaccinated and 1,556 were unvaccinated. Most of the vaccinated app users received either the Pfizer or Moderna vaccines. On average, the first vaccination dose was associated with a 0.71-day increase in a woman’s cycle length. Another dose of COVID vaccine increased the cycle length by 0.91 days.

There were no changes in the number of menstrual bleeding days for the vaccinated patients, and the researchers did not detect any significant change in the cycle length for unvaccinated app users. However, 358 app users who received two doses of the coronavirus vaccine in the same menstrual cycle experienced a markedly larger average increase in their cycle length – lasting up to two days longer.

According to the researchers, this change appeared to decrease in the following cycles, suggesting that the menstrual changes are temporary and will disappear shortly after vaccination. Bianchi and the team add that the International Federation of Gynecology and Obstetrics classifies a change in cycle length as “normal” if the change is less than eight days.

The study is published in the journal Obstetrics and gynecology.

South West News Service writer Georgia Lambert contributed to this report.


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